Back: Measure length of pet’s back from the
base of the neck to the base of the tail. See below for
estimating by breed.Neck: Measure the circumference of pet’s neck
for a snug fit. No need to add inches as you would with a
collar.Chest/Girth: Measure girth around the
broadest part of the chest and add 2”.

Review: I LOVE this product. I ordered it and within 3 days received it. The sizing was spot on, its very warm and that was my main reason for getting it. My pitty baby doesn't have a lot of fur and what he has is very thin so he gets cold very easy but loves to be outside so now he stays warm and happy. I do wish there was a Velcro or zipper but thankfully Lance is very accepting of this and don't try to fight it.